1. Field of the Invention
The present invention relates to an electronic control unit which is placed in, for example, an engine compartment of a vehicle.
2. Description of the Related Art
In an electronic control unit (ECU), for example, used for control of a vehicle, a microcomputer for performing operational processing, an input/output circuit connected to an external load, a sensor, a power supply circuit for supplying power to circuits, and like components have been conventionally placed on a substrate. Then, these circuits and substrate are housed within an enclosure typically consisting of a case and a cover.
The electronic components constituting the above-mentioned circuits generate heat during their operation. An excessively increased temperature of the electronic components adversely affects the operation of the components. Therefore, in order to reduce a temperature of the electronic components, a method for transferring the heat to the substrate, and the like, so as to diffuse the heat is known.
Moreover, as shown in FIG. 12, for an electronic component (for example, a semiconductor chip of a power transistor) P1 that generates a particularly large amount of heat, a method using a radiator fin P2 or the like has been used to efficiently dissipate the heat generated from the electronic component P1 toward a case P3. However, given current product demands, the electronic control unit has been required to have higher function and performance levels, while the heat generated from the electronic component P1 increases.
Accordingly, in order to dissipate a larger amount of heat from the heat-generating electronic component P1, the structure as shown in FIG. 13 has been adopted. In this structure, a large piece of copper foil P6 is placed on the region where the electronic component P1 (more specifically, a heat sink P5) is attached on a substrate P4. The heat is dissipated via holes P7 to other larger pieces of copper foil P8 and the like. In this method, however, since an effective wiring area on the substrate P4 is decreased, the substrate P4 in a large size is accordingly required, leading to an increase in cost.
On the other hand, miniaturization of the electronic control unit is also desirable. In order to respond to such a need for miniaturization, that is, a method for miniaturizing the components in accordance with the development of semiconductor integration techniques, a method for making a number of circuits IC-compatible and the like have been used. However, the use of such methods causes an increase in temperature of the electronic component P1.
As measures against the increase in temperature of the electronic component P1, it has been proposed to use the expensive electronic component P1 which results in little power loss. Additionally, it has been proposed to mount the components on the radiator fin P2 or to increase the size of the substrate P4 to a certain degree so as to improve heat dissipation. However, these methods result in increased costs.
It is also conceivable to make the heat-generating electronic component P1 itself highly heat resistant. However, such a measure is not necessarily preferable because peripheral components placed at a high density also have an increased temperature due to heat transferred from the substrate P4, whereby the size of the substrate P4 must be increased to a certain degree or the peripheral components must be high heat-resistant components.